1. Field of the Invention
The present invention relates generally to optical recording heads that are utilized with optical disk data storage devices, and more particularly to the design and fabrication of a recording head having an optical wavelength resonant cavity media heating device formed therein.
2. Description of the Prior Art
Optical drives generally include one or more rotatable data storage disks having a data storage media layer formed thereon, and data in the form of small areas, termed data bits are written onto the media layer of the disks by a recording head. Optical recording is essentially a thermal process where the thermal energy is delivered to the media optically by means of a small focused spot of light. The size of the spot generally determines the bit size, and the route to higher data areal recording density is to produce smaller spot sizes. The technology of optical recording is outlined below, and is described in numerous references such as “Principles of Optical Disc Systems,” G. Bouwhuis, J. Braat, A. Juijser, J. Pasman, G. van Rosmalen and K. Schouhamer Immink, Adam Hilger Ltd., 1985.
For ease of understanding, a schematic diagram of a typical prior art optical drive is depicted in FIG. 1. As depicted therein, the optical drive 10 includes a rotating media disk 14 upon which data is written by optical head. The optical head includes two parts, a stationary head portion 18 which includes a laser and other components, and a movable head portion 22 which typically includes a laser beam directing mirror 26 and a beam focusing lens 30. In the typical optical drive 10, the movable head 22 is disposed upon a linear actuator and moved radially relative to the rotating disk. The laser beam is directed from the stationary head 18 to the mirror 26 of the moving head 22, through the focusing lens 30 to a spot on the media. With conventional optical systems, the spot size scales directly with the laser wavelength and inversely with the numerical aperture (NA) of the focusing lens 30. These are the two main variables that are changed to reduce the spot size to achieve higher data areal recording density.
There are two main rewriteable optical recording technologies: phase change and magneto-optical. In phase change, localized melting of the disk medium, typically composed of a chalcogenide material, followed by a rapid cooling causes the heated spot on the recording material to change to an amorphous state. Melting followed by a slow cooling process causes it to change to a crystalline state. These two states are designed to have different reflectivities so that the recorded signal can be read out as changes in reflectivity when a reading laser beam is subsequently scanned over the data. In reading, the laser power is lowered so that the state of the medium is not affected, i.e. not melted again. For magneto-optical systems, the optical beam is used to heat the medium above its Curie point as a broad area magnetic field is applied. Only the heated area of the medium responds to the applied field and changes its magnetized state accordingly. Readout is done using polarized laser light whose polarization is changed slightly by the state of magnetization of the medium. The polarization of the reflected light is detected to produce the readback signal.
For non-rewritable media, different techniques can be used. For example in write-once media, the laser spot can be used to locally ablate the medium, thus causing a change in reflectivity. Pre-recorded media (e.g. CD'S and DVD's) use physical pits that are stamped in the surface of the disk to encode the data. The present invention can in principle be used for writing either phase change or ablative write-once or magneto-optical media.
One of the principal features of a current prior art optical recording device is that it is able to focus a small (approximately 0.5 micron diameter) spot on the media 14 at a distance of some millimeters from the movable head 22. This has provided high data storage density without the head-disk-interface issues of magnetic recording. However, the spot size reaches diffraction limits somewhere around 0.2 microns given the diode lasers that are currently available. To achieve smaller spots optically, it is necessary to resort to “super-resolution” techniques. The present invention is essentially a super-resolution technique that achieves a factor of perhaps 10 beyond the diffraction limit, whereas most super-resolution concepts provide much less than that. A trade off for this gain in resolution is that the device must operate in the optical near-field, that is from approximately 1 to 100 nm from the media, such that the head-disk separation is a small fraction of the wavelength of light instead of millimeters. This then requires a flying head as is utilized in normal magnetic recording.
One way to achieve super-resolution is to force the light to go through a small aperture. Here the spot size in the near-field of the aperture can be set by the aperture size rather than being limited by the wavelength of the light as in diffraction limited systems. However, the transmittance of sub-wavelength apertures decreases as r4 where r is the radius of the aperture. Thus the transmittance efficiency of sub-wavelength apertures is very poor and high power lasers would be required to heat the medium. Recent work has investigated C apertures and other special apertures which have higher efficiencies than normal sub-wavelength apertures. The present invention combines sub-wavelength apertures, potentially including C apertures or similar devices, with an optical resonant cavity to amplify the intensity incident on the aperture and thus increase overall efficiency of transmitting light from the laser source to the medium.
In order to understand the operation of an optical resonant cavity it is useful to first consider the resonance of a simple circular cylindrical cavity at microwave frequencies, i.e., the cavity is a hollow circular cylinder. In microwave electronics, a closed circular cylinder has well defined resonances represented by transverse magnetic TMmnp modes and transverse electric TEmnp modes. The indices m, n, and p refer to the number of modes in the azimuthal, radial and longitudinal directions, respectively. For the present case, we limit our discussions to the fundamental mode, TM010, i.e., there are no variations in the azimuthal and longitudinal directions. In this simple mode the magnetic field for TM010 is concentric with the cylinder. On the other hand, the electric field is in the axial direction and has a maximum in the center of the cylinder. All its electric field lines span between the two side walls.
In an effort to increase the electric field in a circular cylindrical cavity, the reentrant cylindrical cavity resonator 112 shown in FIGS. 2A and 2B was developed for the generation of microwave power with klystrons and magnetrons. This cavity 112 is simply a circular cylindrical cavity with a coaxial post 116 which is shorter than the thickness of the cavity. The post extends from one side wall 120 and ends at a subwavelength distance, d, from the opposite side wall 124. An aperture 128, usually of subwavelength diameter, is placed in the side wall 124 opposite to the end of the post 116. The presence of the post 116 compounds the designation of the resonant modes. However, the cavity 112 is normally operated to resonate in a fundamental mode such that the electric and magnetic fields are axisymmetric. Further, the electric field at the post end remains parallel to the axis and peaks at or near the axis. Also, the magnetic field is perpendicular to the electric field such that its field lines are concentric circles about the axis. The presence of the post 116 intensifies the electric field at the axis because the post to side wall separation, d, is now smaller than the thickness of the cylinder. Generally, the smaller the magnitude of d, the greater the axial electric field strength along the axis. What these experiments in the microwave regime have demonstrated is that a reentrant cylindrical cavity of subwavelength dimensions can produce very high intensity electric field in a direction normal to the face 124 of the cavity, a fact that is extended in the present invention for near-field thermal heating at optical wavelengths.
Since any modification to a resonant cavity, such as a post or an aperture, perturbs the simple TE and TM modes in the cavity, in the following we will refer to axial modes where the fields in the original, unmodified resonant cavity would have been TM and will use in-plane modes to refer to fields which would have been TE in the unmodified resonant cavity. Thus for a circular cylindrical cavity, axial fields will imply that the electric field is predominantly oriented parallel to the axis of the cylinder and in-plane fields will imply that the electric field is predominantly perpendicular to the axis of the cylinder and thus in the plane of the air bearing surface.
An important consideration in using an optical resonant cavity for near-field heating is in coupling the optical power into the resonant cavity 112. A known technique in optical communication in coupling power into a circular cylindrical optical cavity is by way of evanescent-wave coupling from an integrated waveguide. As an example of this, R. W. Boyd et al., in Journal of Modern Optics, 2003, Vol. 50, No. 15-17, 2543-2550, “Nanofabrication of optical structures and devices for photonics and biophotonics” teaches a system consisting of a waveguide coupled to a resonant whispering gallery mode (WGM) cavity. The technique is schematically represented in FIGS. 2C and 2D where a tapered planar waveguide 140 is placed near a circular disk microcavity 144. The coupling can be effected if the waveguide 140 is placed with a gap 148 that is a fraction of a wavelength from the cavity. In this device the cavity and waveguide are comprised of a relatively high index of refraction material 150 such as GaAs that is surrounded by a relatively low index of refraction material, in this case air. The upper and lower surfaces of the waveguide and cavity are likewise bordered by layers 152 of relatively low index of refraction material, such as AlxGa1-xAs, where x equals 0.4.
Recently, descriptions of several thermal recording methods for near-field heating of media in the context of thermally assisted magnetic recording have been published. These incorporate an optical heating device in a conventional magnetic recording head. In published U.S. patent applications US2003/0184903 A1 and US2004/0008591 A1 special high transmission near-field apertures disposed within a magnetic head are taught for applications in perpendicular recording. In one example, a near-field aperture is located immediately downtrack of the write pole of the magnetic head such that the input plane of the near-field aperture is parallel to the air bearing surface (ABS). In published U.S. patent applications US2004/0001420 A1 and US2004/0062503 A1 a planar waveguide is constructed on the downtrack side of the write pole of a magnetic head. In general the size of the heated spot depends on the optical wavelength, the aperture dimensions and the composition of the materials for the waveguide/near-field aperture.
As will be understood from the following description, the present invention employs near-field thermal heating for an optical recording medium using a resonant cavity. The application is similar to conventional optical recording systems. However, there are key differences, notably: (1) the heated spot can be as small as 20 nm (compared to about 700 nm in current diffraction limited optical storage devices with a red or near IR laser); (2) since it is limited to near-field heating, there is no focusing mechanism between the optical head and the disk; (3) the read/write bandwidth of this optical head from the perspective of the cavity is about 1 GHz or greater because the estimated time scale to reach resonance in the resonant cavity for a pulsed input is a few hundred femtoseconds.